The present invention relates to a method of forming a multilayer printed circuit board and an apparatus for carrying out the method and, more particularly, to a method of forming a multilayer printed circuit board in the manner that it is free of any voids formed therein wherein a plurality of printed circuit board components and a plurality of prepregs are alternately laminated upon each other and pressed together by a bonding press to form the multilayer printed circuit board and an apparatus for carrying out the method.
A multilayer printed circuit board comprises a plurality of printed circuit board components and a plurality of prepregs alternately laminated upon each other and compressed together by using a bonding press in general, the prepregs being of a bonding resin comprising a polyimide resin, for example.
In other words, the laminated assembly of the printed circuit board components and the prepregs alternately laminated upon each other is sandwiched between an upper jig plate and a lower jig plate before the assembly is integrally pressed and bonded. Occurrence of shifting of positions of the printed circuit board components relative to each other is prevented by using aligning pins. After the laminated assembly of the printed circuit board components is sandwiched between the upper jig plate and the lower jig plate, the upper jig plate and the lower jig plate are clamped between heating plates of a bonding press to raise the temperature of the plates to a predetermined value, and, thereafter, the upper jig plate and the lower jig plate are pressed against each other with the printed circuit board components and the prepregs sandwiched therebetween so as to bond the printed circuit board components and the prepregs with each other.
However, when a multilayer printed circuit board is formed by using a bonding press as described above, particularly, when a plurality printed circuit board components and a plurality of prepregs are laminated alternately and pressed against each other so as to be bonded with each other, voids tend to be generated within the prepregs and, when voids are generated in the prepregs, inferior insulation will take place around throughholes of the printed circuit board thereby not only deteriorating the reliability of the multilayer printed circuit board but also lowering the yield of products.
In consideration of the above, there has been proposed a method of voidless or pore-free formation of a multilayer printed circuit board using a vacuum press as disclosed in a catalogue "JSW-VPI Vacuum Press" published by Kabushikikaisha Nihonseikosho on Apr. 1, 1985.
The method of forming a multilayer printed circuit board by the vacuum press process comprises placing the laminated assembly of the printed circuit board components and the prepregs within a bag before they are integrally pressed and bonded with each other, locating the bag in a high pressure tank, and, thereafter, exhausting the interior of the bag so as to render the interior of the bag to be in vacuum state, and, then, subjecting the bag to an atmosphere of an inert gas of a high temperature and a high pressure from the exterior of the bag so as to complete a multilayer printed circuit board. With such a method, the interior of the prepregs can be effectively made voidless or pore-free.
However, when a multilayer printed circuit board is formed by using a vacuum press, an expensive high pressure tank and a heating and pressurizing apparatus are required, and, further, caution must be taken in manipulating the high pressure tank and the heating and pressurizing apparatus.